DE3784123T2 - METHOD FOR PRODUCING CERAMIC COMPOSITIONS. - Google Patents

Description

This invention relates broadly to a method of making ceramic composites. More particularly, this invention relates to a method of making ceramic composites by infiltrating a filler coated with a silicon source having doping properties with an oxidation reaction product grown from an aluminum base metal precursor.

In recent years, there has been growing interest in using ceramic materials for structural applications that have historically used metals. The impetus for this interest has come from the superiority of ceramic materials over metals in terms of certain properties, such as corrosion resistance, hardness, elastic modulus, and heat resistance.

Current efforts to produce ceramic articles of greater strength, greater reliability and greater toughness are directed primarily toward 1) the development of improved processing techniques for one-piece ceramics and 2) the development of new material compositions, particularly ceramic matrix composites. A composite structure is one consisting of a heterogeneous material, body or article made from two or more different materials that are intimately combined to obtain the desired properties of the composite. For example, two different materials can be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite article contains one or more different types of filler material(s), such as particles, fibers, rods or the like.

Various suitable materials have been used as fillers in the formation and fabrication of composite articles with a ceramic matrix. These fillers have been used in the form of fibers, pellets, particles, whiskers, etc. These materials include, for example, some of the oxides (singly or mixed), nitrides, carbides or borides of aluminum, hafnium, titanium, zirconium, yttrium and silicon. Certain known materials that have been used as fillers, such as silicon carbide and silicon nitride, are not stable as such in a high temperature oxidizing atmosphere (e.g. above 850ºC), but undergo degradation reactions with relatively slow kinetics in such an environment.

There are some known limitations or difficulties in replacing metals with ceramic materials, such as the breadth of the size range, the ability to produce complex shapes, meeting end-use requirements, and cost. Some of these limitations or difficulties have been overcome by several co-pending patent applications owned by the same owner as the present application, which disclose novel methods for reliably producing ceramic materials, including ceramic composite articles. An important process is basically disclosed in EP-A-155831 of the same applicant. This application discloses the process for producing self-supporting ceramic bodies which are grown as the oxidation reaction product of a parent metal precursor. Molten metal is reacted with a vapor phase oxidant to form an oxidation reaction product and the metal migrates through the oxidation product towards the oxidant, thereby continuously building up a ceramic polycrystalline body. The process can be accelerated by the use of an alloyed dopant as used in the case of the oxidation of magnesium and silicon doped aluminum with air to form ceramic structures of α-alumina. This process has then been improved by the application of dopants to the surface of the precursor metal as described in EP-A-169067 of the same applicant.

This oxidation phenomenon has been used to produce ceramic composites, as described in the applicant's EP-A-193292. This application discloses novel methods of producing a self-supporting ceramic composite article by growing an oxidation reaction product from a base metal precursor into the permeable mass of a filler, thereby infiltrating the filler with a ceramic matrix. However, the resulting composite article has no defined or predetermined external shape, form or configuration.

A process for producing ceramic composite bodies of predetermined external shape or form is disclosed in EP-A-245192 (not prepublished) of the same applicant. According to the process of this patent application, the developing oxidation reaction product infiltrates a permeable preform towards a defined surface boundary. It has been discovered that high fidelity is more easily achieved if the preform is provided with a barrier element as disclosed in EP-A-245193 (not prepublished) of the same applicant. This process produces self-supporting ceramic bodies, including shaped ceramic composite articles, by growing the oxidation reaction product of a metal precursor to a barrier element which is positioned remote from the metal to define a boundary or surface. A process for forming ceramic composite articles having a cavity with an internal geometry that inversely repeats the shape of a positive base metal mold or pattern is disclosed in EP-A-234704 of the same applicant.

The present invention broadly provides a method for producing a ceramic composite body consisting of a ceramic matrix obtained by the oxidation reaction of molten aluminum base metal with an oxidizing agent including a vapor phase oxidizing agent and a filler which is at least initially filled with a silicon-containing precursor or CVD-deposited silicon coated and infiltrated by the matrix. The applied coating is different in composition from the primary composition of the filler, and the silicon-containing precursor coating may be at least partially reduced or dissolved from the molten parent metal under the process conditions. This coating, when heated to a suitable temperature, preferably but not necessarily in an oxygen-containing environment, intrinsically possesses doping properties for accelerating the oxidation reaction, and the substantially unchanged or residual portion of the filler serving as the filler is incorporated into the developing matrix, as described in more detail below.

The self-supporting ceramic composite body is made by first forming a bed or mass of filler material, some or all of the components being coated with a silicon-containing precursor. The coating differs in composition from the primary composition of the filler. The filler may be at least partially overlaid with a barrier element which is at least partially spaced from the aluminum base metal to define a surface or boundary of the ceramic matrix.

The coated filler, which may be used with other filler materials, either in the form of a build-up, a packed bed, or as a preformed preform, is placed or oriented adjacent to the aluminum parent metal so that formation of the oxidation reaction product occurs in a direction toward the oxidant and filler, and toward the barrier element, if one is used. The bed of filler material or preform should be sufficiently permeable to permit or accommodate growth of the oxidation reaction product in the bed and to permit the gaseous oxidant to penetrate the preform and contact the molten metal. The parent metal is heated to a temperature above its melting point but below the melting point of the oxidation reaction product, forming a body of molten metal. At this temperature, or range of temperatures, the molten metal reacts with the oxidant to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the molten metal and the oxidant to draw molten metal through the oxidation reaction product toward the oxidant and into contact with the oxidant such that the oxidation reaction product continually forms at the interface between the oxidant and previously formed oxidation reaction product, thereby infiltrating the adjacent filler material. The reaction is continued for a time sufficient to infiltrate at least a portion of the filler material with a polycrystalline material consisting essentially of the oxidation reaction product and one or more metallic constituents, such as non-oxidized constituents of the parent metal or the dopant, distributed or dispersed in the polycrystalline material. It is understood that the polycrystalline Material may contain voids or porosity in place of the metal phase, but the volume percent of voids depends largely on such conditions as temperature, time, dopants, and type of base metal. If a barrier element has been used, the ceramic body will continue to grow up to the barrier layer, provided that sufficient metal is present.

As explained in the aforementioned patent applications of the same applicant, the use of dopant materials can beneficially influence or promote the oxidation reaction process. Silicon is a useful dopant for an aluminum parent metal, especially in combination with other dopants, and it can be applied externally to the parent metal, and one useful source of such a dopant is silicon oxide. Under the process conditions of this invention, a silicon-containing compound as a silicon source (e.g., silicon oxide) is reduced by the molten parent metal to form aluminum oxide and silicon. Thus, the coating of the silicon-containing compound on the filler material represents a useful dopant for promoting the development or growth of the oxidation reaction product. One useful filler material is, for example, silicon carbide coated with a suitable silicon-containing compound as a precursor to the silicon source, which may be a silicon oxide film. The silicon oxide film is reduced by the molten aluminum parent metal and yields a silicon dopant which promotes the growth of the polycrystalline matrix through the silicon carbide filler. In addition, the silicon oxide coating on the silicon carbide particles is advantageous because it increases the local silicon concentration in the non-oxidized silicon parent metal during the matrix formation reaction, thereby reducing the tendency to form Al₄C₃ during the matrix growth process. Al₄C₃ is undesirable because it is unstable in the presence of moisture levels normally found in ambient air, leading to the formation of methane and degradation of the structural properties of the resulting composite body.

It has been found for the purposes of this invention that by carrying out the oxidation reaction, preferably in an oxygen-containing environment, the applied coating of silicon-containing precursor can be converted into a silicon source which serves as a dopant material for the oxidation reaction of the parent metal. The remaining part of the filler beneath the coating, which is of a different composition, is substantially unchanged and serves as a filler in the composite body. For example, the filler may carry a silicon-containing compound which can be reduced by the molten metal, or the filler may be CVD-coated with silicon which can be dissolved by the molten metal. It is to be understood that substantially all of the silicon source may be used as a dopant, or only a portion of it may be used as a dopant, the remainder remaining in the filler and embedded by the matrix. According to the invention, a separate coating material is applied to the filler which upon heating forms the silicon source. The coating of the silicon source, e.g. a silicon-containing compound, may be formed or prepared by first firing or heating a suitable filler in an oxygen-containing atmosphere. The firing filler bearing a coating is then used as a filler material. For example, a preform may be prepared from particulate silicon carbide or particulate alumina coated with a silicon-containing precursor or compound such as tetraethyl orthosilicate. The preform is then firing or heating in air to form an oxide skin of silicon oxide on the silicon carbide or alumina particles. The preform may then be used as a green ceramic composite having within it a source of silicon dopant material. Alternatively, the silicon carbide or alumina particulate material may be used with the silicon-containing coating in the filler material-base metal assembly, and the silicon oxide film or coating is formed in situ during the process of oxidation reaction in the presence of an oxygen-containing gas. The primary composition of the particulate filler (e.g., silicon carbide or alumina particulate) remains intact and serves as the filler material for the composite body.

The materials of this invention can exhibit substantially uniform properties across their cross-section, to a thickness heretofore difficult to achieve using conventional methods of producing ceramic structures. The process that produces these materials also avoids the high costs associated with conventional methods of producing ceramic materials, including the preparation of fine, homogeneous, high purity powders and their densification by such methods as sintering, hot pressing or isostatic pressing.

The products of the present invention are adaptable or manufactured for use as articles of commerce, which, as used herein, are intended to include, without limitation, industrial, structural and engineering ceramic bodies for such applications in which electrical, wear-resistant, thermal, structural or other features or properties are important or advantageous; and the term is not intended to include recycled or waste materials which may be generated as unwanted byproducts in the processing of molten metals.

The terms mentioned below are defined in this specification and the accompanying claims as follows:

"Ceramic" is not intended to be strictly limited to a ceramic body in the classical sense, that is, in the sense of being composed entirely of non-metallic and inorganic materials, but rather refers to a body which is predominantly ceramic either in terms of its composition or in terms of its predominant properties, although the body contains minor or major amounts of one or more metallic elements derived from the parent metal or formed from the oxidizing agent or by a doping agent. components, most typically within a range of approximately 1-40 percent by volume, but which may contain more metal.

"Oxidation reaction product" generally means aluminum as the parent metal in any oxidized state in which the metal has donated or shares electrons with another element, compound, or combination thereof. Accordingly, an "oxidation reaction product" under this definition includes the product of the reaction of the aluminum metal with an oxidizing agent such as those described in this application.

"Oxidant" means one or more suitable electron acceptors or electron sharing substances and may be a solid, a liquid or a gas (vapor) or a combination of these (e.g. a solid and a gas) under the process conditions.

"Parental metal" refers to aluminum which is the precursor to the polycrystalline oxidation reaction product and includes relatively pure aluminum, commercially available aluminum containing impurities and/or alloying constituents, or an alloy of aluminum in which aluminum as the precursor is the major constituent or the most important constituent for the formation of the oxidation reaction product.

"Silicon source" refers to elemental silicon or a silicon compound that provides a dopant material and/or promotes wetting of the filler by the molten base metal under the process conditions.

About the pictures:

Figures 1, 2 and 3 are top and side views, respectively, of a composite body prepared according to Example 2. In each of these figures, a portion of the grown composite body was removed by cutting for further investigation.

Figure 4 is a micrograph taken at 50x magnification of a composite structure prepared according to Example 3 showing coated filler particles embedded in a ceramic matrix.

In carrying out the process of the invention, the aluminum parent metal, which may be doped with an additional dopant material (as explained in more detail below) and which is the precursor to the oxidation reaction product, is formed into a billet, block, rod, plate or the like. A mass or body of filler material comprising particles, powders, fibers, whiskers or other suitable shapes and having a coating of a silicon source is positioned relative to the aluminum parent metal such that growth of the oxidation reaction product occurs toward and into the filler. The coating applied differs in composition from the primary composition of the filler. It is believed that the silicon source also serves to improve the wettability of the filler by the parent metal. The bed is permeable to the vapor phase oxidant (e.g. air) and for the growth of the oxidation reaction product matrix to permit development of the oxidation reaction product and thereby infiltration of the filler. As explained in commonly owned patent applications, dopant materials beneficially affect the oxidation reaction process of parent metals, and silicon, silicon dioxide and similar silicon-containing compounds are useful sources of dopants in systems employing aluminum as the parent metal. According to a preferred embodiment of this invention, a silicon-containing compound, when heated to an appropriate temperature in an oxygen-containing atmosphere, forms an oxide coating which serves as a source of dopant material. Formation of the oxide coating on the filler can be accomplished in a firing step or in situ during formation of the ceramic body in the presence of an oxygen-containing gas as an oxidant. Unless otherwise stated, the terms "filler" or "filler material" are intended to mean either a mass, bed or preform consisting of the filler material at least partially coated with a silicon source and which may be used in combination with other filler materials not having such a coating.

As explained above, the filler comprises a suitable coating of a silicon-containing precursor applied to a filler of entirely different composition. For example, a particularly useful system of this type consists of tetraethyl orthosilicate applied to zirconia fibers, which upon drying or mild heating to dissociate the material forms a coating of silicon oxide. As another example, a glass of ethyl silicate, which upon heating forms a coating of silicon oxide, can be applied to particulate alumina.

According to the invention, the filler material can also be coated with silicon by chemical vapor deposition (CVD). This can be particularly useful in the case of fillers that are typically present as fibers, particles or whiskers that need to be protected from degradation under the process conditions. For example, boron nitride particles need to be protected from oxidation and reaction with molten aluminum, and the silicon CVD coating provides this protection while still meeting the other criteria of the invention.

The filler materials, such as silicon carbide and silicon nitride, are preferably in particle form and may contain a mixture of different grit and mesh sizes, preferably from about 2 mm - 12 um (10-1000 mesh), but finer particles may also be used. However, in the case of silicon nitride, it is desirable to use a relatively coarse material to prevent excessive oxidation or reaction to form aluminum nitride and silicon. In this way, the blended filler can be tailored to have the desired final properties, such as permeability, porosity, density, etc.

The filler is typically combined into a bed or preform with any suitable binder, agent, compound or the like that does not interfere with the reactions of the invention or leave a significant amount of undesirable residual byproducts in the ceramic composite product. Suitable binders have been found to include, for example, polyvinyl alcohol, epoxy resins, natural or synthetic latex, and the like, which are well known to those skilled in the art. The filler, with or without a binder, can be formed into a predetermined shape and size by any conventional method, such as slip casting, injection molding, transfer molding, vacuum forming, etc.

Preferably, the filler material should be preformed to have at least one surface boundary and should retain sufficient shape integrity and green strength and shape fidelity during processing and formation of the ceramic body. However, the filler bed or preform should be sufficiently permeable to accommodate the growing polycrystalline material of the matrix. For example, a silicon carbide or silicon nitride preform useful in this invention has a porosity of between about 5 and about 90 volume percent, and more preferably between about 25 and 75 volume percent.

In carrying out the process, the coated filler, which may be fired and/or preformed, is placed adjacent to one or more surfaces or a portion of a surface of the aluminum. The filler material is preferably in surface contact with a surface of the base metal; but if desired, it may be partially immersed in the molten metal, but not completely submerged, since complete immersion would cut off or block access of the vapor phase oxidant to the filler material necessary for proper development of the polycrystalline matrix. Formation of the oxidation reaction product occurs in a direction toward and into the filler material.

The assembly consisting of the filler and the aluminum base metal is placed in a furnace charged with a suitable vapor phase oxidant and the assembly is heated to a temperature range or within a temperature range above the melting point of the base metal but below the melting point of the oxidation reaction product. The process temperature range for an aluminum base metal using air as the vapor phase oxidant is generally about 700-1450°C, or more preferably about 800-1350°C. Within this operable temperature interval or preferred temperature range, a body or pool of molten metal forms and upon contact with the oxidant, the molten metal reacts to form a layer of oxidation reaction product. With continued contact with the oxidizing environment, the remaining molten metal, within the proper temperature range, is progressively drawn through and into the oxidation reaction product and toward the oxidant. Upon contact with the oxidant, the molten metal to form additional oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the molten parent metal and the oxidant, thereby causing continued transport of the molten metal by the formed oxidation reaction product toward the oxidant such that the polycrystalline oxidation reaction product infiltrates at least a portion of the filler material. The silicon source coating formed on the filler material by precoating and, if desired, firing promotes the growth of the polycrystalline oxidation reaction product by providing a continuous source of the silicon dopant material throughout the volume of the filler.

The process is continued until the oxidation reaction product has infiltrated at least a portion of the bed of filler material. If a preform is used, then the process is continued until the oxidation reaction product has infiltrated and embedded the constituents of the preform up to a specified surface boundary, and preferably not beyond which would constitute "overgrowth" of the polycrystalline matrix material.

It will be understood that the resulting polycrystalline matrix material may have porosity which may represent a partial or nearly complete replacement of the metal constituents, but the volume percent of voids will depend significantly on such conditions as temperature, time, parent metal type and dopant concentrations. Typically in these polycrystalline ceramic structures the oxidation reaction product crystallites are interconnected in more than one dimension, preferably three dimensions, and the metal or pore constituents may be at least partially interconnected. The resulting ceramic composite product has the dimensions and geometric configuration of the original preform, if one was used, and a particularly good match is achieved by the use of a barrier element.

The vapor phase oxidant used in the oxidation reaction process is normally in gaseous or vaporized form under the process conditions, creating an oxidizing atmosphere, e.g. ambient air. However, if a fired or precoated filler is used, then the oxidant need not be an oxygen-containing gas. Typical vapor (gaseous) oxidants, the use of which may depend on whether the filler has been fired or precoated, further include, e.g., nitrogen or a nitrogen-containing gas and mixtures such as air, H₂/H₂O and CO/CO₂, the latter two (i.e., H₂/H₂O and CO/CO₂) being useful for reducing the oxygen activity of the environment toward desired oxidizable constituents of the preform. Oxygen or oxygen-containing gas mixtures (including air) are suitable vapor phase oxidants, with air usually being more preferred for obvious economic reasons. When a vapor phase oxidizer is specified to be a specific gas or vapor, then it means an oxidizer in which the specified gas or vapor is the sole, predominant, or at least a major oxidizer of the parent metal under the conditions prevailing in the oxidizing environment employed. For example, although the major constituent of air is nitrogen, the oxygen content of the air is the sole oxidizer of the parent metal under the conditions prevailing in the oxidizing environment employed. Air therefore falls within the definition of an "oxygen-containing gas" type oxidizer, but not within the definition of a "nitrogen-containing gas" type oxidizer. An example of a "nitrogen-containing gas" type oxidizer as used herein and in the claims is "forming gas," which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen.

An oxidizer that is liquid or solid at the process conditions may be used along with the vapor phase oxidizer. Such additional oxidizers may be particularly useful for preferentially accelerating the oxidation of the parent metal within the filler material rather than outside its surfaces. That is, the use of such additional dopants may create an environment within the filler material that is more favorable to oxidation kinetics than the environment outside the filler bed or preform. With respect to the silicon carbide filler material used as a preform, this improved environment is beneficial for promoting the development of the matrix within the preform to the limit and minimizing excess growth.

If a solid oxidizer is used in addition to the vapor phase oxidizer, it may be distributed throughout the volume of the filler material or only in a portion of the filler material adjacent to the base metal, e.g. in the form of particles mixed with the filler material. Any suitable solid oxidizer may be used, depending on its compatibility with the vapor phase oxidizer. Such solid oxidizers may include suitable elements, such as boron, or suitable reducible compounds, such as certain borates, borate glasses, silicates and silicate glasses of lower thermodynamic stability than the oxidation reaction product of the base metal.

When a liquid oxidizer is used in addition to the vapor phase oxidizer, the liquid oxidizer may be distributed throughout the volume of the filler bed or a portion adjacent to the molten metal, provided that such liquid oxidizer does not prevent access of the vapor phase oxidizer to the molten parent metal. Reference to a liquid oxidizer means one that is liquid under the conditions of the oxidation reaction, and so a liquid oxidizer may have a solid precursor, such as a salt that is molten or liquid under the conditions of the oxidation reaction. Alternatively, the liquid oxidizer may be a liquid precursor, such as a solution of a material that is molten under the process conditions.

melted or decomposed to form a suitable oxidizing species. Examples of liquid oxidizing agents as defined here include low melting point glasses.

If a shaped preform is used, then the preform should be sufficiently porous or permeable to allow the vapor phase oxidant to penetrate the preform and contact the molten parent metal. The preform should also be sufficiently permeable to accommodate the growth of the oxidation reaction product within its confines without disturbing, disrupting or otherwise altering its configuration or external shape. In the event that the preform contains a solid oxidant and/or a liquid oxidant that may accompany the vapor phase oxidant, then the preform should be sufficiently porous or permeable to allow and accommodate the growth of the oxidation reaction product resulting from the solid and/or liquid oxidant.

When one or more dopants are used in addition to the silicon source, they (1) may be available as alloying constituents of the aluminum parent metal, (2) may be applied to at least a portion of the surface of the parent metal, or (3) may be applied to or incorporated into all or a portion of the filler material, or any combination of two or more of techniques (1), (2), or (3) may be used. For example, a dopant alloyed into the parent metal may be used alone or in combination with a second externally applied dopant together with the silicon-containing compound coating. In the case of technique (3), in which one or more additional dopants are applied to the filler material, the application may be done in any suitable manner, as explained in the patent applications of the same applicant. The function or functions of a dopant material may depend on various factors and not only on the dopant material itself. Such factors include, for example, the particular combination of dopants when two or more dopants are used, the use of an externally applied dopant in combination with a dopant alloyed into the base metal, the concentration of the dopant, the oxidizing environment and the process conditions.

Dopants useful in combination with a silicon source as a dopant for an aluminum base metal, particularly with air as the oxidant, include magnesium and zinc, which may be used in combination with other dopants as described below. These metals, or a suitable source of the metals, may each be alloyed into the aluminum-based base metal in concentrations between about 0.1-10 weight percent based on the total weight of the resulting doped metal. If desired, the silicon metal may be alloyed with the base metal to supplement the silicon source present as a coating on the filler. In such examples, a preferred concentration of the magnesium is in the range of about 0.1 to about 3 weight percent, the silicon is in the range of about 1 to about 10 weight percent, and of zinc, when used with magnesium, in the range of about 1 to about 6 weight percent. These dopant materials or a suitable source thereof (e.g., MgO or ZnO) may also be used externally of the base metal. Thus, an alumina ceramic structure can be made from the aluminum base metal using air as an oxidant by using MgO as a dopant in an amount greater than about 0.0008 g per gram of base metal to be oxidized or greater than 0.003 g per square centimeter of base metal to which the MgO is applied.

Other examples of dopant materials effective with aluminum base metals that react with an oxygen-containing atmosphere include sodium, germanium, tin, lead, lithium, calcium, boron, phosphorus, and yttrium, which may be used individually or in combination with one or more other dopants, depending on the oxidant and process conditions. Rare earth elements such as cerium, lanthanum, praseodymium, neodymium, and samarium are also useful dopants, and again especially when used in combination with other dopants. All of these dopants, as explained in the patent applications of the same applicant, in addition to the coating from a silicon source, are effective in promoting the growth of the polycrystalline oxidation reaction product for aluminum-based base metal systems.

The ceramic composite obtained using the present invention is typically a coherent product in which between about 5 and about 98 volume percent of the total volume of the ceramic composite consists of filler embedded in a polycrystalline ceramic matrix. The polycrystalline ceramic matrix typically consists, when air or oxygen is the oxidizing agent, of about 60 to about 99 volume percent (of the volume of the polycrystalline matrix) of coherent alpha-alumina and about 1 to 40 weight percent (same basis) of metallic constituents such as unoxidized constituents of the parent metal or reduced metal from the dopant or oxidizing agent.

As described in EP-A-245193 (not prepublished), a barrier element may be used in conjunction with the filler material to inhibit the growth or development of the oxidation reaction product beyond the barrier. A suitable barrier element may be any material, compound, element, composition or the like which, under the process conditions of this invention, maintains some integrity, is non-volatile and preferably is permeable to the vapor phase oxidant, while at the same time being capable of locally inhibiting, poisoning, stopping, affecting, preventing or the like the continued growth of the oxidation reaction product. Calcium sulfate (gypsum), calcium silicate and Portland cement and mixtures thereof, which are particularly useful with aluminum as the parent metal and an oxygen-containing gas as the oxidant, are typically applied as a slurry or paste to the surface of the filler material. These barrier elements may also contain a suitable combustible or volatile material which disappears on heating, or a material which decomposes on heating, to increase the porosity and permeability of the barrier element. Furthermore, the barrier element may contain a suitable heat-resistant particulate material to reduce any shrinkage or cracking which may otherwise occur during the process. Such particulate material having substantially the same coefficient of expansion as the filler bed is particularly desirable. For example, if the preform comprises alumina and the resulting ceramic body comprises alumina, then the barrier element may be mixed with alumina particles which most preferably have a particle size of about 840-12 µm (20-1000 mesh).

The following non-limiting examples illustrate the practice of certain aspects of the invention.

example 1

In accordance with the present invention, a ceramic structure was prepared consisting of an oxidation reaction product of alumina embedding whiskers of β-SiC supplied by Nikkei Techno-Research Company Ltd. and first coated with either a commercially available colloidal silica (Ludox HS-30 from Du Pont Company) or a solution of sodium silicate (40-42° Baume) as an additional silicon source.

Three preforms, 5.08 cm (2 in) diameter and 9.53 mm (3/8 in) thick, were prepared by mixing three separate batches of β-SiC whiskers with a liquid medium, pouring the resulting slurry into a mold, and then degassing and drying it in a vacuum desiccator. The liquid media mixed with the β-SiC whiskers consisted of distilled water as a control, colloidal silicon oxide, and a sodium silicate solution. The preforms were placed on a bed of 216 µm (90 grit) particle size E1 Alundum (from Norton Company) contained in a refractory boat. Aluminum alloy blocks (No. 712.2) of the same diameter as the preforms were coated on one side with a thin layer of sand, and the coated side of each block was placed in contact with the top surface of a preform. This assembly was placed in a furnace and heated to 900°C in 5 hours. This temperature was maintained for 36 hours, and the assembly was cooled to ambient temperature in 5 hours. In the preform containing only the β-SiC whiskers (the control using distilled water), infiltration by the alumina oxidation reaction product was negligible. The β-SiC whiskers coated with colloidal silica were infiltrated throughout the thickness of the preform. Infiltration of the β-SiC whiskers with the sodium silicate solution occurred to approximately the middle of the preform.

Example 2

In accordance with the present invention, a ceramic composite structure was prepared consisting of an oxidation reaction product of alumina embedding particles of a silicon carbide filler material [39 Crystolon, 17 µm (500 grit) from Norton Co.] which were first coated with colloidal silica (Ludox HS-30 from Du Pont Company, 30% solution) as a silicon source.

Coating of the silicon carbide particles with the colloidal silica was accomplished by preparing two 5.08 x 5.08 x 1.27 cm (2 x 2 x 1/2 in) preforms by sediment casting a mixture of 17 µm (500 grit) silicon carbide particles and colloidal silica in a ratio of 2 parts powder to 1 part liquid into a rubber mold. After settling and drying, one of the preforms was crushed and 100% passed through 0.14 mm (100 mesh) screen openings. The crushed colloidal silica coated silicon carbide was then sediment cast again using a 2% acrylic latex binder (Elmer's Wood Glue, Bordon Co.). For comparison, a silicon carbide preform identical to the above preform, not coated with colloidal silica, was prepared using only the latex binder.

Three bars of aluminum alloy 712 (which had a nominal composition, in weight percent, of 0.15% Si, 0.6% Mg, and 6% Zn) were placed in a refractory bed of wollastonite fibers (Nyad FP from Petty-Rowley Chemical Co.) contained in a refractory vessel so that a 5.08 x 5.08 cm (2 x 2 in) surface area of each bar was exposed to the atmosphere and substantially flush with the bed. One of each of the three preforms described above was placed on the tops of the alloy bars so that a 5.08 x 5.08 cm (2 x 2 in) surface area of each preform and alloy were substantially superimposed. A layer of wollastonite fibers was spread on top of the preforms to reduce excess growth of the ceramic matrix beyond the confines of the preform. This assembly was placed in a furnace and heated to 1000ºC in 10 hours. The furnace was held at 1000ºC for 80 hours and then cooled to ambient temperature in 10 hours. The assembly was removed from the furnace and the resulting ceramic composite structures were removed. The resulting composites were lightly sandblasted to remove unembedded preform materials. Figures 1(a), 1(b), 2(a) and 2(b) are photographs of the resulting composites obtained using preforms with the colloidal silica coating (Figures 2(a) and 2(b) show a newly cast preform) illustrating good growth; and Figures 3(a) and 3(b) show the resulting composite after no silica was used. As can be seen from the figures, the preforms formed from the silica coated particles were embedded essentially to their mold boundaries, while the preform containing no silica showed significantly less infiltration through the ceramic matrix.

Example 3

According to the present invention, a ceramic composite structure was prepared consisting of an oxidation product of alumina embedding particles of boron nitride coated with silicon.

A rod of aluminum alloy 380.1 (from Belmont Metals, with a reported nominal composition in weight percent of 8-8.5% Si, 2-3% Zn and 0.1% Mg as active dopants and 3.5% Cu plus Fe, Mn and Ni, but the Mg content was sometimes higher, such as in the range of 0.17-0.18%) was immersed in a bed of boron nitride particles [of approximately 297 µm (50 mesh) size]. The boron nitride particles were coated with silicon (achieved by chemical vapor deposition) to protect the boron nitride particles from degradation and to serve as a source of silicon dopant to supplement the silicon source in the alloy. This bed was contained in a refractory boat. This assembly was placed in a furnace, which had an opening to allow access of air, and heated to 1100ºC over a period of 5 hours. The furnace was maintained at 1100ºC for 48 hours and then cooled to ambient temperature. The resulting ceramic composite was removed. Figure 4 is a micrograph of the composite taken at 50x magnification, showing the matrix 2 of aluminum oxide embedding the boron nitride particles 4, which still carry some silicon coating 6.

Claims (13)

1. A process for producing a self-supporting ceramic composite material comprising (1) a ceramic matrix obtained by oxidizing an aluminum base metal to form a polycrystalline material comprising (a) an oxidation reaction product of a base metal with at least one oxidizing agent including a vapor phase oxidizing agent and optionally (b) one or more metallic constituents, and (2) a filler infiltrated by said matrix; the process comprising: (A) orienting said base metal and a filler material relative to each other such that formation of the oxidation reaction product occurs in a direction toward and into said filler, said filler providing at least one oxidation reaction promoting dopant for the aluminum base metal, (B) heating said aluminum parent metal to a temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten aluminum parent metal, and reacting the molten aluminum parent metal with the oxidizing agent at said temperature to form said oxidation reaction product, and at said temperature maintaining at least a portion of said oxidation reaction product in contact with and between said body of molten metal and said oxidizing agent to progressively draw molten metal through the oxidation reaction product toward the oxidizing agent and into the filler material so that fresh oxidation reaction product continues to form within said filler at the interface between the oxidizing agent and previously formed oxidation reaction product; and (C) continuing said reaction for a time sufficient to infiltrate at least a portion of said filler with said polycrystalline material, characterized in that at least a portion of said filler material comprises (i) a coating of silicon applied by chemical vapor deposition, or (ii) a coating of a silica-derived precursor applied to the filler by coating the filler with a suspension or solution of said silica-derived precursor and drying it, said silica-derived precursor being capable of forming a silicon source having dopant properties, or (iii) a coating of a silicon source formed from said coating of the silica-derived precursor by its oxidation or dissociation, said silica-derived precursor and said silicon source having a composition different from the basic composition of the filler. 2. The method of claim 1, wherein the chemical vapor deposition silicon coating is present as a coating on a boron nitride filler material. 3. The method of claim 1, wherein the coating is made of a silica-derived precursor selected from a group consisting of colloidal silica, a silicate, tetraethyl orthosilicate and ethyl silicate glass. 4. The method of claim 1 or 3, wherein the filler is selected from the group consisting of silicon carbide, silicon nitride, zirconium oxide and particulate alumina. 5. A process according to any one of claims 1, 3 and 4, in which said suspension or solution of the silica-derived precursor further contains a binder, and in which a filler bed or a shaped filler preform is formed which is dried. 6. A process according to any one of claims 1 and 3 to 5, wherein the coating of silica-derived precursor is a silica-derived compound reducible from said molten aluminum parent metal in steps (B) and (C). 7. The process of claim 1, wherein said oxidation or dissociation to form said coating of said silicon source is carried out prior to the orientation step in (A). 8. The method of claim 1, wherein said oxidation or dissociation to produce said coating from said silicon source is carried out in situ during the formation of said oxidation reaction product. 9. A process according to any one of claims 7 or 8, wherein said oxidation to form said coating is carried out by heating said coating of silica-derived precursor in the presence of an oxygen-containing atmosphere to form a coating of silicon oxide. 10. A method according to any one of the preceding claims, wherein at least one additional dopant material is used in conjunction with said aluminum base metal. 11. A method according to any one of the preceding claims, which comprises forming said filler material into at least one preform having at least one defined surface boundary. 12. The method of claim 11, additionally comprising overlaying the preform with a barrier element for inhibiting the formation of said oxidation reaction product thereabove. 13. A process according to any preceding claim, wherein said oxidizing agent additionally comprises at least one of a solid oxidizing agent or a liquid oxidizing agent or both incorporated into at least a portion of said filler material, said process further comprising reacting said molten aluminum parent metal with said additional oxidizing agent, and said polycrystalline material further comprising the oxidation reaction product of said aluminum parent metal with said additional oxidizing agent.